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Re: Explaining raising conditions and defining condition types in the gu
Re: Explaining raising conditions and defining condition types in the guile manual
Fri, 9 Nov 2018 14:16:16 -0500
On Fri, Nov 9, 2018 at 11:10 AM Ludovic Courtès <address@hidden> wrote:
The R6RS exception mechanism is close to but different from SRFI-34/35,
> and I presume the R7RS mechanism is likewise close to but different from
> both R6 and SRFI-34/35. :-)
Sorta. The *exception* system is the same as in R6RS except that `error`
does not use the `who` argument (staying compatible with SRFI 23).
But no *condition* system are defined, so that implementations
can keep their native condition systems. (Racket wound up with two
condition systems, native and R6RS.)
Consequently, portable programs can catch a condition with `guard`
or `with-exception-handler`, but can't discriminate between different
exceptions, with three, umm, exceptions:
`read-error?` says if a condition reflects an error is signaled by `read`
indicating bad S-expression syntax.
`file-error?` says if a condition reflects a file that cannot be opened or
These cases are precisely those where R5RS says "an error must be signaled".
(Well, there is also passing a bad number to `scheme-report-environment`,
but who cares.)
Also, `error-object?` returns true on conditions created and raised by
but may return true on other conditions as well, implementation-dependently.
I'm hoping to add more predicates of this type for R7RS-large. The
of such a feature cannot be portable, but should be trivial in any one
John Cowan http://vrici.lojban.org/~cowan address@hidden
Newbies always ask:
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